13.证明:若f(x)在[a,b]上连续,且 lt (x)_(1)lt (x)_(2)lt ... lt (x)_(n)lt b ,则在[x1,xn]上-|||-必有点ξ,使得 (xi )=dfrac (f({x)_(1))+f((x)_(2))+... +f((x)_(n))}(n) -